The present invention relates to a method and apparatus for increasing welding rates for high aspect ratio (high depth to width ratios) welds, and more particularly, for increasing welding rates for high aspect ratio welds formed between similar or dissimilar crack sensitive workpieces using a filler wire.
Many traditional welding processes use a wire feed to supply a filler wire. However, and in applications where crack sensitive materials are involved, these processes typically apply too much heat and accordingly lead to unwanted defects.
In other processes, the wire is an auxiliary feed wire (a tungsten inert gas, or TIG process). However, these processes require more time.
For one current cladding process technique that is used to minimize defect formation, the crack sensitive alloys are maintained at an elevated temperature during the complete cladding process. However, when rebuilding the tip of a gas turbine airfoil (e.g. GTD111) this process could take several hours at elevated temperatures. Moreover, this is a manual process, which does not provide an operator friendly environment.
Other materials, such as IN738, are cycled between low and high temperatures to avoid having the component at elevated temperatures too long and thus, minimize defects and component distortion. However, such a cyclic process extends the processing times required to affect the required clad buildup.
Several prior art, welding processes employ laser welding techniques. For example, U.S. Pat. No. 4,634,832, (Martyr), assigned to British Shipbuilders, forms butt welds or T welds using a laser-induced plasma to weld adjacent walls of the plates to be welded. More particularly, the process of Martyr focuses a laser beam on a beam-interceptor material to form a plasma. The plasma is held in place by a gas supply such that the plasma transfers energy into the walls and thereby melts the faces, welding the two plates. Although this high energy laser welding process can be used to produce high aspect welds, it is unsuitable for welding crack-sensitive materials because the plasma would not only transfer excessive heat to the crack-sensitive materials to be welded but would further embed the beam interceptor material into the crack sensitive materials. Both the excessive heat and the embedding would cause defects and cracking.
U.S. Pat. No. 4,737,612 (Bruck et al.), assigned to Westinghouse Electric Corp., forms laser keyhole welds without filler material. This process is not desirable for crack sensitive materials because the energy transfer from the laser (having a power density of about two to five million watts per square centimeter) to two adjacent crack sensitive materials would cause cracking in the materials. Bruck et al. further discloses forming a laser conduction weld by passing a laser along the sides of the confronting surfaces to be joined and passing a resistively heated filler wire into the resulting metal pool. The latter process is also undesirable for crack sensitive materials and for the purposes of the invention for the following reasons. First, direction of a laser beam at the walls of two adjacent crack sensitive components to form a molten pool, would transfer excessive heat to the components, inducing cracks and other defects into the crack-sensitive materials. Further, Bruck et al. form a conduction weld. As known to those skilled in the art, conduction welds have low depth to width ratios and typically have distortion along the weld. Typical aspect ratios for conduction welds are less than or equal to one. Thus, the latter method of Bruck et al. does not produce the desired high aspect ratios of the present invention.
Another prior art laser welding process is disclosed in U.S. Pat. No. 4,803,334 (Burke et al), assigned to Westinghouse Electric Corp. Burke et al. form a conduction weld with a shallow pool of molten metal joining two, abutting metal matrix composite components by oscillating a laser beam across the intersection thereof. A preheated filler wire is fed into the molten pool to add filler material to the weld. Like the process of Bruck et al, this process is undesirable for crack-sensitive materials because direction of a laser beam at the walls of two adjacent crack sensitive components to form a molten pool, would transfer excessive heat to the components, inducing cracks and other defects into the crack-sensitive materials. Further, conduction welds have low aspect ratios and are prone to distortion.
Commonly assigned U.S. Pat. No. 5,958,261 (Offer et al.) describes an electric arc or laser welding process for producing high aspect ratio welds, in which a filler wire is fed into a groove extending above a work surface and defined by two opposing sidewalls. Although the tip of the filler wire is melted by the arc and/or the superheated weld puddle (depending on the wire aim position), and the groove is preferably narrow enough that cross seam oscillation is unnecessary, the remaining arc heat is primarily transferred directly to the workpiece, rather than to the filler wire, because the projected area of the diverging arc onto the workpiece is several times greater than the projected area onto the relatively thin wire. Thus, as expressly disclosed, the method of Offer et al. would be undesirable for use with crack-sensitive substrates because of the heat transfer to the substrates.
Accordingly, it would be desirable to develop a laser welding process employing a wire feed for producing high aspect ratio welds that can be used to join similar and dissimilar crack sensitive substrates.
In a first embodiment of the present invention, a method for lasing a filler wire to weld a first workpiece to a second workpiece is disclosed. The first and second workieces comprise first and second materials, respectively.
A tip of the filler wire is positioned above an opening defined by the first and second workpieces. The tip extends over a width of the opening and over a portion of the first workpiece and a portion of the second workpiece.
The filler wire is preheated. A laser beam is directed at the tip of the filler wire to melt it. The laser beam is advanced along a length of the opening with the tip of the filler wire being positioned under the laser beam to form a high aspect ratio weld between the first and second workpieces. The high aspect ratio weld has an aspect ratio of at least about two.
In a second embodiment of the present invention, an apparatus for laser welding a first workpiece to a second workpiece, the workpieces defining an opening, is disclosed.
A filler wire has a diameter that exceeds a width of the opening. A wire feed device is configured to continuously supply the filler wire. A guide is configured to direct a tip of the filler wire to a position above the opening to extend over the width of the opening and over a portion of the first workpiece and a portion of the second workpiece.
A laser is configured to direct a laser beam at the tip of the filler wire for forming a high aspect ratio weld between the first and the second workpieces. The high aspect ratio weld has an aspect ratio of at least about two. A power supply is configured to supply a current to the filler wire.